New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming provincewith Laurentia

A question regarding the 1900-1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne provin ce (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogen y, or; a separate entity welded later to a Hearne-Superior continent (centr al Canada). New Ar-40/Ar-39 mica dates help to address this question by ext ending a Middle Proterozoic geochronologic front, long established along th e southern Wyoming province, into the Black Hills of South Dakota. This sug gests that previously unexplained, north-directed fold nappes (F-1) in the Black Hills resulted from island-are accretion to the south ca. 1780 Ma. No rth-northwest-trending upright F-2 folds, which formed during east-west col lision of the Wyoming and Superior provinces, must therefore be younger. Ne w Ar-40/Ar-39 hornblende dates, recently published age data, and crustal he at-flow considerations further suggest that this collision began at or befo re ca, 1770 Ma and culminated with posttectonic magmatism beginning ca, 171 5 Ma (the Harney Peak granite). This tectonic-magmatic interval is similar to 50-60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810- 1710 Ma) also typify the eastern and northern Wyoming province periphery (w estern Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior provi nce (and to each other) during distinct Early Proterozoic orogenies, Region al relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca, 1780-1740 Ma interval of islan d-arc accretion along the southern margin of the growing craton.